Carburetor idling too high after new fuel pump

[Bullwinkle]

After replacing my mechanical fuel pump the old Quadrajet carb is idling way to high.  I would guess around 2,200 rpm.  With the adjustment screw backed all the way off.  This is a 79 350, all original.  The pump is the same as the one I took off except its a little later model, with the return line to the tank, for vapor lock protection.  I'm thinking I should put a pressure gauge on the supply line going to the carb.  I guess that means putting a T in that line? 
Many thanks. 

[bd]

Make sure that the choke fast idle cam isn't stuck on the high step.  Then use some aerosol carburetor cleaner and spray for vacuum leaks.

[JohnnyPopper]

Do you have a return line?

Having one in the first place would suggest that there is an internal pressure regulator that routes gas back to the tank when not consumed by the carb/engine.

Or it just runs a higher volume to provide gas to the carb and return line.

So if you don't have a return line that may be the problem.

Why didn't you get a correct model?

A pressure test would tell you a lot, and yes, you can tee the line between the pump and carb.

[bd]

JP, the mechanical fuel pump with a return port incorporates a small diameter restriction in the return nipple of the pump to limit the volume of fuel returning to the tank(s).

Do you have a return line?

Having one in the first place would suggest that there is an internal pressure regulator that routes gas back to the tank when not consumed by the carb/engine.

Or it just runs a higher volume to provide gas to the carb and return line.

So if you don't have a return line that may be the problem.

How?   

[rodnrudy]

  Make sure you haven't inadvertently pulled a vacuum line off somewhere. That will cause a high idle.

[JohnnyPopper]

bd I know you're having fun with me, but assuming the pump has a return tube and there is no return line to the tank, what else is he going to do but cap it off?

Please enlighten us 

[bd]

Well, hypothetically, let's say the return line is capped off, ... or not.  How might either result in a high idle on a carbureted engine? 

[VileZambonie]

You probably knocked the choke wire off or a vacuum line popped off.

[Bullwinkle]

Thanks for the great suggestions, I’ll check the choke.  It idles the same speed hot or cold, so I didn’t check it but yes it could be stuck all right.   
Yes there’s a return line to the tank, that’s why I got that type.   I heard a guy say it worked well for him without any modification of anything else.
I don’t see how a return line could increase pressure unless the pressure is set higher on the supply line than on the older two-line pump.   
   If I do end up putting in a gauge, what pressure would that original quadrajet use? 
Thanks again very much

[Bullwinkle]

The reason I put in the later 3-port pump was the reason they designed it in the first place.  Returning some fuel to the tank continuously keeps it from getting hot enough to vapor lock. 

[Rapid Roy]

Well, hypothetically, let's say the return line is capped off, ... or not.  How might either result in a high idle on a carbureted engine? 

X2

Should always replace parts that are designed for the year and model, especially if it is original engine!

[bd]

For the 1979 model year trucks, small block fuel pressure for engines using a return line is 5 1/2 - 7 PSI; small block engines without a fuel return line call for 7 1/2 - 9 PSI.  Big block engines with a fuel return line call for 7 1/2 - 9 PSI.

Bullwinkle, fuel pressure isn't the cause of the high idle complaint.  Most likely either the choke isn't releasing or air is bypassing the throttle (e.g., a vacuum leak).

JP, I was just trying to lead you to think the symptoms through.  I know you have the knowledge and experience.   

[JohnnyPopper]

bd=mentor

Thanks!